If you can't remember your password, you can change it by:
Starting from your OS X install CD while holding the letter 'C'
At the first screen (choose a language), first go to the Installer menu (at the top of the screen) and choose reset password, and select your user name from the menu. Then you change the password (use something you can remember). Then quit that Reset Password app, and continue with the OS X installer. At the screen where you choose your drive, click on the options button, and select the Archive and Install option. You'll find that you can now re-install OS X!
Re-installing OS X is usually a choice far down the list if you are having problems. (This is not MS Windows, you know
) Why do you think you need to re-install?